This ensures the open issues/MR count caches are refreshed properly when
creating new issues or MRs. This MR also includes a change to the cache
keys to ensure all caches are rebuilt on the fly.

This particular problem was not caught in the test suite due to a null
cache being used, resulting in all calls that would use a cache using
the underlying data directly. In production the code would fail because
a newly saved record returns an empty hash in #changes meaning checks
such as `state_changed? || confidential_changed?` would return false for
new rows, thus never updating the counters.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38061